Molds for forming articles and in particular, molds in which rubber tires are valcanized, are usually provided with a plurality of strategically located small diameter passages or vent holes extending from the mold cavity interior to the exterior. These vent holes bleed or discharge trapped air and gas from within the mold during vulcanization of the tire to prevent the formation of air or gas bubbles in confined areas of the mold preventing the flow of rubber into these areas, in order to completely form the final molded product and in particular the side wall, shoulder and tread areas of a rubber tire. The vent holes are normally machined or drilled through the walls of the mold and are provided with some type of filter which permits the passage of the air or gas therethrough without allowing appreciable amounts of rubber to flow through or into the mold vents. Although it is desirable to completely reduce the flow of rubber into the vents to avoid forming flash, such flow does occur resulting in a number of "runners" or rubber flashings being formed on the final molded product or tire. These runners require a finishing operation for the removal of the excess rubber or runners from the tire surface. The runners, depending upon their size and thickness, must be removed from the finished tire. In other molds, the runners will be torn from the tire surface upon removal of the tire from the mold causing an inperfection in the tire surface and plugging or closing the vent openings for subsequent molding operations.
To reduce the formation of runners on molded articles, including tires, various types of plugs have been devised which are inserted into the vent openings which permit the passage of the gases and air therethrough without passage of appreciable rubber. Some examples of these prior art mold vents and in particular vents having types of vent opening plugs are shown in the following patents.
U.S. Pat. No. 3,891,363 discloses the use of Teflon inserts which are inserted into the venting holes to provide anti-adhesive properties to prevent rubber from sticking to the inserts and to permit air to freely discharge from the mold while preventing rubber from fully clogging the vent system.
U.S. Pat. No. 3,822,857 discloses the use of a synthetic resin vent hole plug in the molds for the molding of rubber products. The resin plugs have no adherence to rubber and the air is vented out through a plurality of round microventing holes formed in the plug. The plugs are formed of a nonadhering synthetic resin such as tetrafluorothylene resin.
U.S. Pat. No. 3,804,566 discloses a tire mold having a vent filter of a sintered body containing a plurality of linear air venting holes. The small diameter of the holes in the filter material will result in extremely small rubber projections or runners.
U.S. Pat. No. 4,447,197 discloses a tire mold having a plurality of air vent plugs formed of a plastic material with a low coefficient of friction, such as polysulfone. The plugs are selected of materials which are adapted to prevent creep and swell during use of the mold.
Japanese Patent No. 45-5897 discloses a tire vulcanizing mold with vent plugs comprises of an outer cylindrical portion made of stainless steel having a central plug or rod of a polytetrafluoroethylene.
Australian Patent No. 218,991 discloses a tire mold having a porous sintered stainless steel ring positioned within the vent hole which is covered with a coating of polytetrafluoroethylene (Teflon).
German publication DOS No. 2808474 discloses air permeable elements for molds in which the element is formed of PTFE which completely fill portions of vent nozzles which extend through the mold wall.
Although some prior art mold vents and in particular the vent plugs therefor, do reduce the formation of runners, many of these known prior art plugs are susceptible to clogging and have a relatively short life span requiring continual replacement. Furthermore, such vent plugs cannot withstand mold blasting and require replacement after each mold cleaning operation. Also, some of these prior art mold vent plugs require cement or other means for mounting the plugs in the vent holes increasing the cost of their installation in the mold. This is important for each mold and such increased cost can be considerable over the life of the mold.
Furthermore, all of these prior art vent plugs are generally flush with the inner article forming surface of the mold in an attempt to reduce the formation of runners and thus are "passive" plugs in contrast to the "active" plug of my invention described more fully below. Therefore, the need has existed for an improved mold vent plug which eliminates difficulties encountered with prior vent plugs and achieves desirable results, and in particular, the elimination or reduction of runners of molded rubber on a final molded tire or other product.